Generally, a plasma is used to etch silicon of a semiconductor wafer (hereinafter, simply referred to as “wafer”). However, a plasma for etching silicon needs to be generated from chlorine (Cl2) gas, hydrogen bromide (HBr) gas or the like, which requires a high environmental load and a high cost. Therefore, it is required to etch silicon without using a plasma in view of a low environmental load and a low cost.
As for a method for etching silicon without using a plasma, there is suggested a method using fluorine (F2) gas and nitrogen monoxide (NO) gas or a method using nitrosyl fluoride (FNO) gas. Specifically, there is known a method for removing a silicon deposit by simultaneously supplying fluorine gas and nitrogen monoxide gas (see, e.g., Japanese Patent No. 4739709).
However, when fluorine gas, nitrogen monoxide gas and nitrosyl fluoride gas are simultaneously supplied, all the gases act as etching species and, thus, simultaneous etching by the respective etching species occurs. Accordingly, it is difficult to control an etching rate, a shape after etching, or the like. Recently, it has been confirmed that an etching shape is changed when a temperature of the wafer is changed in the case of simultaneously supplying fluorine gas, nitrogen monoxide gas and nitrosyl fluoride gas (see, e.g., Tajima Satoshi et al., “MEMS sacrificial layer Si etching without using a plasma”, NanotechJapan Bulletin Vol. 7, No. 4, 2014, “Nanotechnology EXPRESS” the 26th Special Issue, 1-5. Specifically, when the temperature of the wafer is 230° C. or above, the etching rate is increased but it is easy to control the shape after the etching due to surface orientation. When the temperature of the wafer is lower than 230° C. and higher than or equal to 60° C., the etching rate is decreased and the controllability of the shape after etching is improved. Therefore, in the case of supplying a plurality of gases acting as etching species, a desired etching shape can be obtained by controlling a temperature of the wafer.
However, when the temperature of the wafer is lower than 60° C. which is a practical temperature, an agglomerated layer of fluorine, nitrogen monoxide and nitrosyl fluoride is generated, and the respective etching species in the agglomerated layer react with silicon at different etching rates. Thus, the etching rate is considerably increased and the shape after etching becomes poor. In other words, in the case of simultaneously supplying a plurality of gases acting as etching species, it is difficult to appropriately control the etching rate or the shape after etching.